It is usual to equip apparatus under pressure such as boilers, with safety valves to avoid overpressures. The oldest and most rudimentary method of doing this consists in providing a safety valve formed by a valve member pressed against its seat by means of a calibrated spring, thus sealing the vessel to be protected. When the internal pressure of the vessel exceeds a predetermined value the valve member is lifted against the pressure of the spring. Such a method has the disadvantage that the valve rattles against its seating. Furthermore the valve member does not open completely, so that it produces a throttling effect on the fluid escaping from the vessel, resulting in erosion of the components. Because of rattling, the operation of such a valve is noisy. It is also to be noted that such valves require special maintenance, notably in relation to the biasing springs of the valve member which frequently require changing.
Another solution consists in associating the valve mounted on the vessel to be protected with an assembly comprising a cylinder within which is slidably mounted a piston having its rod fastened to the valve member. Said cylinder is in communication with the fluid contained in the vessel to be protected, a device for sensing the pressure and controlling the operation of the valve assembly being interposed in the communication duct.